A charge pump is a circuit that can generate an output voltage that is higher than the voltage supplied to the charge pump. One of the applications of charge pumps is to develop voltages for various modes of operation of flash EPROM (electrically programmable read only memory). The memory cells in flash EPROM are formed using so-called floating gate transistors in which the data is stored by charging or discharging the floating gate. These modes of operation generally require a high voltage.
The act of charging the floating gate is termed the "program" mode of a flash EPROM. This is typically accomplished using hot electron injection induced by establishing a large positive voltage between the gate and source, as much as 12 volts, and a positive voltage between the drain and source, for instance, 7 volts.
The act of discharging the floating gate is called the "erasing" mode of a flash EPROM. This erase function is typically carried out through Fowler-Nordheim tunneling induced by establishing a large positive voltage from the source to gate, while floating the drain of the memory cell. This positive voltage can be as much as 12 volts.
Some prior art flash EPROM devices require the application of an external high voltage in addition to a regular 3 or 5 volts supply voltage. Such an arrangement is undesirable because it complicates system design. Recently, many nonvolatile semiconductor memory device manufacturers (including flash EPROM manufacturers) place charge pumps on chips to generate the required high voltage for erasing and programming memory cells.
The output voltage of a charge pump needs to be regulated. Prior art systems use breakdown devices (such as zener diodes) or equivalent circuits that become conductive when voltage applied thereto exceeds a predetermined value. Although this arrangement can keep the output voltage below the predetermined value, a large amount of current needs to flow through the breakdown device. As a result, prior art regulator systems consume much power.
Because it is a low power system design, it is important to keep power consumption down. Consequently, there is a need to design a more efficient regulator system which consumes less power.